The present invention relates to a method of and a system for controlling a valve operating mechanism for an internal combustion engine having a plurality of intake or exhaust valves openable and closable by rocker arms angularly movable by cams rotatable in synchronism with the crankshaft of the engine.
There is known a valve disabling mechanism in an internal combustion engine for changing the number of intake and exhaust valves in operation dependent on the rotational speed of the engine for thereby optimizing intake and exhaust efficiencies throughout the entire engine speed range from low to high speeds (see Japanese Laid-Open Patent Publication No. 61-31614). Japanese Laid-Open Patent Publication No. 61-19911, filed by the same assignee as the assignee of the present application, has proposed a method of increasing the efficiency of charging an air-fuel mixture into a combustion chamber in a wide engine operating range by varying the timing of operation of valves differently in low and high engine speed ranges.
In the known arrangements, a solenoid-operated valve is actuated dependent on the rotational speed of the engine to open or close a hydraulic circuit for moving a piston disposed in one rocker arm to connect or disconnect the rocker arm to or from an adjacent rocker arm, so that the rocker arms can be operated in unison with or separately from each other. To allow the piston to move smoothly, it is necessary that the adjacent rocker arms be held at rest, i.e., held in sliding contact with base-circle portions of respective cams when the piston is moved.
Generally, after a control signal is applied to the solenoid-operated valve to shift its spool for opening the hydraulic circuit when the rotational speed of the engine reaches a preset speed, a considerable delay is experienced, dependent on the oil pressure and oil temperature, before the piston is actually operated. Moreover, while the adjacent rocker arm is swinging in a direction to open the associated valve, the piston cannot easily be extended into the adjacent rocker arm. It will be apparent, therefore, that during the initial stage of the valve opening cycle, even if the piston is successful in entering the bore in the adjacent rocker arm slightly, the piston will be released from, or struck, by the rocker arm before it completely enters the bore to effect full engagement, thus producing noise or being damaged. On the other hand, the connected rocker arms while they are in swinging motion cannot reliably be released from each other because of frictional engagement between the piston and the bores in the respective rocker arms. Therefore, it is apparent from the above that the rocker arms may not smoothly be coupled to, and released from, each other dependent on the particular timing of operation of the piston.
According to the valve disabling mechanism shown in Japanese Laid-Open Patent Publication No. 61-31614, the piston which is extendable and retractable is normally urged by a spring housed therein in a direction to extend, the piston and a timing plate is provided which is engageable with the piston for controlling the timing of operation of the piston. This mechanism is however structurally complex and highly expensive. Therefore, where such mechanism is to be combined with each of the sets of the rocker arms for all engine cylinders, the cost of manufacture becomes very high.
Another method would be to control the opening and closing of the hydraulic circuit for moving the piston such that it occurs during periods except those in which it would be struck by the rocker arm, in view of the delay in operation caused by the hydraulic circuit, even if the engine rotational speed has already reached the preset speed. One problem with this method, however, is that in a multicylinder engine, pistons of the valve operating mechanism would be struck by rocker arms at different times for respective engine cylinders. Since such different times come one after another as the crankshaft or cams rotate, the piston will necessarily be struck with respect to a certain engine cylinder.